add_force

This keyword is used to add force on atoms in a selected group at each step during a run.

Syntax

This keyword is used in one of the following two ways:

add_force <group_method> <group_id> <Fx> <Fy> <Fz> # usage 1
add_force <group_method> <group_id> <add_force_file> # usage 2

  • Force is added to atoms in group group_id of group method group_method.

  • In the first usage, the constant force with components Fx, Fy, and Fz is added on each selected atom.

  • In the second usage, a series of forces specified in the file add_force_file will be periodically added to each selected atom. The file format is:

    N
Fx(0) Fy(0) Fz(0)
Fx(1) Fy(1) Fz(1)
...
Fx(N-1) Fy(N-1) Fz(N-1)

    The first line contains the number of force vectors \(N\), which must be at least 2. It is followed by \(N\) lines, each containing the three Cartesian components of one force vector. At the initial force evaluation (\(t=0\)), vector 0 is used. At \(t=n\Delta t\), vector \(n \bmod N\) is used, so the sequence is repeated periodically.

  • Force is in units of eV/Å.

Example 1

Add a constant force of 0.1 eV/Å in the x direction on atoms in group 1 of group method 2:

add_force 2 1 0.1 0 0

Example 2

Add force on atoms in group 2 of group method 0 using a sequence of force vectors from add_force.txt:

add_force 0 2 add_force.txt

For example, the following file applies three force vectors periodically:

3
0.10 0.00 0.00
0.00 0.10 0.00
0.00 0.00 0.10

Note

This keyword can be used multiple times during a run.